The entire disclosure of Japanese Patent Application No. Hei 8-295716 filed on Oct. 16, 1996 including the specification, drawings and abstract is incorporated herein by reference.
1. Field of the Invention
The present Invention relates to a steel member surface treatment wherein a hardened layer is formed at the surface of the steel member while producing minimal thermal strain.
2. Description of the Related Art
In steel members having surface portions in sliding contact with other members, various measures have conventionally been taken to improve the abrasion resistance of the sliding contact surface portions.
A hard steel provides abrasion resistance but, since forming processes are difficult with hard steel, hard steel cannot be suitably used for a member that requires much forming, for example, a lockup piston or the like.
Accordingly, for steel members requiring considerable forming, typically, only a surface layer is quenched for hardening to improve abrasion resistance. Such a surface hardening method may be surface quenching by high density energy beam irradiation, such as high frequency quenching, electron beam (EB) quenching, laser quenching and the like.
In such quenching methods, a surface to be treated is first heated by high frequency heating or high density energy beam irradiation, and a surface layer portion is maintained at an austenitizing temperature (quenching temperature). When the surface layer is austenitized, the heating is stopped. Then, the steel member is rapidly cooled by, for example, allowing it to self-cool in the ambient atmosphere, so that austenite in the surface layer portion transforms into martensite, thereby forming a hardened layer.
However, in the above-described conventional surface quenching method, since uniform austenite is obtained by heating, it is necessary to maintain a surface layer at a quenching temperature for at least the length of time needed for austenitic transformation. This problem will here be explained with reference to the T-T-A curve diagram shown in FIG. 1. The diagram shows an A3 transformation starting line (austeriitic transformation starting line) and an A3 transformation ending line (austenitic transformation ending line), with the abscissa indicating time (logarithmic scale), and the ordinate indicating temperatures In the diagram, temperature of a steel member surface undergoing a conventional surface quenching method is indicated by a solid line C1. As can be seen from: the diagram of FIG. 1, after heating is started, some time must elapse before a normal structure (ferrite-pearlite structure) will completely transform into austenite in the conventional method.
Therefore, if the member being treated is, for example, a thin plate component, a large portion of the treated member undergoes temperature increase due to heat conduction during austenitic transformation, creating various problems. For example, thermal strain may alter the shape of the member (loss of precision of shape), or self release of heat (spontaneous cooling) may be insufficient to provide proper quenching.
Furthermore, since the conventional method requires maintaining a high temperature for a period of time equal to or longer than the austenitic transformation time as described above, there are also problems of a lengthy heat treatment and low productivity.
The present invention has as its objective overcoming the above-described problems with the conventional treatments. It is intended to provide a steel member surface treatment method which eliminates thermal strain and quenching failure, even if the member under treatment is a thin plate component, and to provide a highly efficient process.
Accordingly, the present invention provides a surface treatment of steel wherein only a surface layer of the steel member is heated to its melting point or higher by high density energy beam irradiation, and then the melted portion is rapidly cooled to a temperature within the martensitic transformation region so as to form a martensitic structure.
Thus, in the present invention, only a surface layer of a steel member is heated to form a melt and the melted portion is converted into a martensitic structure. That is, instead of waiting for completion of austenitic transformation while maintaining a bulk piece at a temperature within an austenitic transformation temperature region as in the conventional art, the melted portion is formed by actively heating a limited area on the surface of the piece at a rapid rate to a temperature equal to the melting point or higher, which temperature is also equal to or higher than the austenitic transformation temperature, to form a martensitic structure by transition from the austenitic structure.
The xe2x80x9chigh density energy beamxe2x80x9d may be, for example, an electron beam, a laser beam, or other high density energy such as high frequency heating, although technically not a xe2x80x9cbeamxe2x80x9d. These are collectively referred to as xe2x80x9chigh density energy beamsxe2x80x9d in the description of the present invention which follows.
Steels which may be treated by the method of the present invention, include, for example, carbon steels such as S50C, S23C, S1OC and the like, alloy steels such as SNCM, SCR, SCM and the like, and tool steels such as SK, SKH, SKS and the like. The aforementioned melting point and martensitic transformation region are determined by the nature of the steel treated.
In the present invention, only a surface portion of a steel member is heated to its melting temperature or higher to form a melted portion by high density energy beam irradiation as described above. Since the thermal energy is provided by high density energy beam irradiation, it becomes possible to very rapidly form a melted portion. Furthermore, since the thermal energy is high density energy, it becomes possible to selectively melt only a surface layer of the steel member in a very short time.
The melted portion is then allowed to rapidly release heat and cool, by stopping the high density energy beam irradiation or by shifting the position of the beam. The melted portion rapidly cools because it is limited to a surface layer of the steel member as stated above. Because the interior of the steel member around the melted portion is at a temperature significantly lower than that of the melted portion, the melted portion rapidly releases heat and thus rapidly cools due to heat conduction to the surrounding portions of the steel member. It is also possible to utilize forced cooling, such as water cooling or the like, in addition to spontaneous heat release.
During the rapid cooling of the melted portion, the melted layer solidifies and immediately obtains an austenitic structure. The austenitic structure is subsequently cooled to a martensitic transformation region in a very short time.
Due to formation of a martensitic structure, the melted portion becomes very hard and forms an excellent hardened. surface.
Thus, the present invention first forms a melted portion only in a surface layer of a steel member in a very short period of time, and then martensitizes the melted portion in a very short period of time. Therefore, it becomes possible to obtain a sufficiently quench-hardened layer and to reduce the surface treatment time and, therefore, to improve productivity. Furthermore, since heat conduction to portions of the steel member surrounding the treated portion is low, temperature rise and thermal strain in the surrounding steel are lower than with the conventional prior art treatment.
The present invention provides a highly efficient surface hardening treatment, with reduced thermal strain and occurrence of quenching failure, even if the steel member is a thin plate component.
In addition, it is possible to provide a heat--treated portion as a smooth finished surface, without any waviness, by suitably selecting depth and width of the melted areas and the processing rate as described below.
It is preferable that the rate of temperature increase in the surface layer of the steel material be equal to or greater than 7500xc2x0 C./second. If the temperature increase rate is less than 7500xc2x0 C./second, problems occur related to increased heat conduction to the steel surrounding the treated portion and increased treatment time. The upper limit for the heating rate is preferably 500,000xc2x0 C./second, in consideration of the practical limits of the apparatus used.
It is also preferred that the time between start of impingement with the high density energy beam and formation of the melted portion be within 0.2 second. If it exceeds 0.2 second, the heat conduction to the metal surrounding the treated portion increases, thereby causing problems of increased thermal strain, due to temperature increase in the surrounding metal, and failure of quenching due to insufficient self-release of heat. The lower limit is preferably 0.003 second, in consideration of the practical limits of the apparatus used.
It is preferable.that cooling to the martensitic transformation region from the melt be at a rate equal to or greater than 600xc2x0 C./second. If the cooling rate is less than 600xc2x0 C./second, there may be a failure of quenching, depending on the type of steel. The upper limit is preferably 1800xc2x0 C./second, from the viewpoint of minimizing thermal strain.
It is preferred:that the melted portion have a depth such that waviness is not produced on the surface of the steel material. More specifically, it is preferred to adjust the output of a high density energy beam, irradiation duration and the like in accordance with the width of the melted portion, the processing speed and the like so that no waviness occurs on the steel member surface. It thereby becomes possible to obtain a steel member with excellent shape precision.
The melted portion may comprise a completely melted layer, i.e. in a completely melted state, and an incompletely melted layer contiguous to the completely melted layer. The incompletely melted layer is a layer that is hardened by quenching based on heat conduction from the completely melted layer, and the quenching depth thereof can be controlled in accordance with the temperature increasing rate. Therefore, a relatively large quenching depth can be obtained without deepening (thickening) the completely melted layer, thereby making it possible to prevent surface waviness.
The high density energy beam may be emitted from a single source of beam emission and divided for impingement upon a plurality of areas, for example, by using a deflecting lens or the like. Thus, a plurality of locations on the steel member can be simultaneously irradiated with high density energy beams, providing surface treatment of a plurality of areas in one processing step. In this manner, heat conduction to the metal surrounding the melted portion can be limited so that even if a plurality of adjacent locations are simultaneously treated, there is no thermal interference among the individual treated regions, and no undesirable tempering or annealing will occur in the treated regions.
It is preferable that the melted portion(s) be rapidly cooled by leaving it to cool by itself, i.e. spontaneous cooling. That is, the cooling is preferably accomplished by mere heat release from the melted portion to the interior and exterior of the steel member. Cooling in this manner provides a simplified operation, as compared with the case of forced cooling, such as water cooling.
It is preferable that the heat capacity of the entire steel member be at least 4 times as large as the heat capacity of the melted portion to provide rapid heat release from the melted portion to the interior of the steel member.
It is also preferred that the depth of the melted portion be at most b {fraction (1/4)} of the thickness of the steel member to provide rapid heat release to the unmelted, surrounding metal.